The present invention relates generally to a system for dispensing liquids, and, more particularly, to an automatic microprocessor controlled system for dispensing a specific amount of liquid to a piece of processing equipment adapted for receiving a specific amount of liquid from the system, and which system automatically switches between a plurality of liquid supply sources that furnish liquid to the system for dispensing when one of the liquid sources is detected as empty.
For certain manufacturing processes it is imperative to carefully regulate the operating conditions of the various processes. One problem encountered in certain manufacturing processes such as those dispensing liquid photoresist chemicals is discontinuities formed in photoresist layers due to air bubbles that are introduced into the process as the liquid photoresist is dispensed. The liquid photoresist chemicals are furnished in relatively small bottles, and it is necessary to replenish the chemicals as they are emptied.
It is known to use pumping as described in U.S. Pat. No. 4,601,409 to create a reduced pressure within a reservoir by pumping liquid from the reservoir to dispense liquid chemicals which concurrently draws liquid from a container into the reservoir to keep the reservoir filled above a pre-set level. From this disclosure, it is also known to switch from one container to another when one container becomes empty, thus allowing filling of the reservoir to continue. However, this is not entirely satisfactory because both filling and dispensing operations are active at the same time. Consequently, if the reservoir cannot be filled at least at the same rate that liquid is being dispensed due to empty inlets, other fluid blockage or other reasons, there is a possibility of gaseous fluid being dispensed along with or in lieu of the liquid chemicals.
Second, the prior pumping arrangement is conventionally provided by a bellows type pump. However, this is not entirely satisfactory because the flow rate is not steady. Consequently, it is difficult to dispense a specific amount of liquid chemicals. Further, with this type of pumping arrangement, if a filter were included with the system, it would be difficult to compensate for the increasing pressure drop across the filter as the filter becomes loaded with contaminants over the passage of time.